This invention relates generally to polycarbonate articles, and more particularly to polycarbonate articles exhibiting improved abrasion resistance.
Polycarbonate resins are well-known, commercially available materials possessing physical and chemical properties which are useful in a wide variety of applications. Because of their excellent breakage resistance, polycarbonates have replaced glass in many products, such as automobile headlamps and stoplight lenses; safety shields in windows, architectural glazing, and the like. However, a major defect exhibited by polycarbonates is their very low scratch-resistance and susceptibility to ultraviolet light-induced degradation.
Methods for improving the scratch-resistance of thermoplastics such as polycarbonate have involved disposing an inorganic protective layer on the surface of the polycarbonate. For example, in U.S. Pat. No. 4,328,646, issued to Kaganowicz, an abrasion-resistant article is formed by subjecting a mixture of hard coating precursors to a glow discharge, and depositing the product directly on a plastic substrate as a very thin film. However, the absence of an intermediate layer between the substrate and the hard coating layer often results in surface cracking when the article is exposed to various heating/cooling cycles.
U.S. Pat. No. 4,200,681, issued to Hall et al., disclosed the vapor deposition of a top layer of glass (SiO.sub.2) onto an intermediate primer layer which in turn has been deposited on the surface of a polycarbonate substrate. However, the evaporative technique of applying a layer of SiO.sub.2 is often undesirable for several reasons. For example, the individual particles of SiO.sub.2 may evaporate and later condense on the coating surface at rates which vary with the particular site of deposition, resulting in a nonuniform glass surface often characterized by pits, pinholes, and other imperfections. Furthermore, this technique generally permits only line-of-site deposition, and curved or indented surfaces would thus exhibit undesirable variations in glass coating thickness. Moreover, evaporative deposition methods like those of Hall et al. do not generate reactive film-forming species which can react with the underlying surface to form a very adherent coating. These methods also require extremely low operating pressures which are sometimes difficult to maintain during deposition, and cause undesirable outgassing of the polycarbonate.
U.S. Pat. No. 3,713,869, issued to Geffcken et al., teaches the deposition of an intermediate layer polymerized by glow discharge onto a polycarbonate surface. A hard inorganic layer is then vaporized by an electron beam gun onto the intermediate layer in a manner similar to that used by Hall et al., and having the same deficiencies discussed above.
There is thus a continuing interest in improved methods for forming abrasion-resistant polycarbonate articles having still greater abrasion resistance, while also exhibiting improvements in various other physical properties.
It is therefore an object of the present invention to provide a method for forming polycarbonate articles having a high level of abrasion resistance.
It is another object of the present invention to provide an improved method for applying smooth, hard, transparent layers of uniform thickness over a polycarbonate substrate.
It is yet another object of the present invention to provide an abrasion-resistant polycarbonate article having disposed thereover a top layer characterized by uniform thickness, high abrasion resistance, and freedom from pinholes and microcracks.